Light emitting diodes (LEDs) provide light in response to receiving a DC current (assuming proper bias) and in proportion to the received DC current. Resistance of an LED light source fluctuates. Therefore, constant current driver circuits are preferred with LED based light sources. Underwriters Laboratories (UL) class II standards for LED driver circuits require certain criteria to pass regulation, including for example that the driver circuit have an isolated output, pass a short circuit test, provide a controlled (i.e., limited) output voltage, and provide a constant current. Of particular importance to the present disclosure is a requirement that at any time the output power cannot be more than 100 watts. A DC-to-DC power converter is one example of a constant output voltage driver that can be configured to control the power output, for example by controlling the output current and voltage at the same time in order to make sure the output power is less than 100 watts at any time.
In an illustrative example, a DC-to-DC converter drives a DC load such as, for example, one or more light-emitting diodes (LEDs). The converter includes a DC-to-AC inverter in a primary circuit. The inverter generates a switched AC voltage, which is applied to the primary winding of an isolation transformer. A secondary winding of the isolation transformer provides an input to an AC-to-DC rectifier in a secondary circuit. The rectifier produces a DC voltage, which is applied to the load. The magnitude of a current flowing through the load is sensed and compared to a reference magnitude. Additionally, the magnitude of a voltage across the load is sensed and compared to a reference magnitude. A first operational amplifier or other comparator in the secondary circuit generates a current feedback signal in response to a difference between the sensed current magnitude and the reference magnitude. A second operational amplifier or other comparator in the secondary circuit generates a voltage feedback signal in response to a difference between the sensed voltage magnitude and the reference magnitude. The feedback signals are fed to a current control circuit to regulate a frequency control current flowing from a control terminal of a switch controller integrated circuit in the DC-to-AC inverter. The switch controller is responsive to the frequency control current to vary the frequency of the switched AC voltage. Varying the frequency of the AC voltage varies the magnitude of the load current to control the output power to be, for example, less than 100 watts.
An issue with controlling the output power by monitoring both the load current and the load voltage is that if the maximum load current is high, for example, greater than 5 amperes, then sensing the current will be very lossy. Accordingly, a special current sensing resistor would have to be used for circuits which produce such a high current. The special current sensing resistor can be very costly.
Additionally, the current feedback signal has to be integrated with voltage feedback signal to generate the frequency control current in the current control circuit, which can be very complicated.